This invention relates to 5,10-dialkyl substituted 5,10-dideazaaminopterin and a cyclized derivative thereof and to the use of such compounds as antineoplastic agents. Another aspect of this invention relates to an improved process for the preparation of 10-ethyl-10-deazaaminopterin.
Methotrexate (MTX) remains the only classical antifolate in established clinical use, and its use has continued to expand as new methods of administering the drug have been introduced and as other tumor types have been added to the list of those now being treated. MTX usage, however, suffers major limitations due to its toxic side effects and the development of resistance by tumor cells. Some tumors are naturally resistant to MTX while others acquire resistance after a period of response. Three factors known to contribute to drug resistance are (a) loss of the active-transport system by which MTX enters cells, (b) increased levels of dihydrofolate reductase (DHFR) the intracellular target of MTX, and (c) the presence of structurally altered DHFR having lower affinity for MTX. Another explanation of resistance may be offered in the recent description of a structurally altered DHFR from an MTX-resistant mutant cell line with unaltered affinity for MTX, but with greater capacity to reduce dihydrofolate than the DHFR from the parent MTX-sensitive cell line. MTX and aminopterin (AMT) have the following structures: ##STR1##
As part of program aimed toward the identification of new antifolate agents that exert greater therapeutic effectiveness against a broader spectrum of tumors than agents now available, antifolates are sought having favorably altered transport characteristics but still possessing tight binding affinity for DHFR. In studies aimed toward greater understanding of transport properties, differences have been observed between tumor and normal proliferative tissue in mediated cellular membrane transport of antifolates and in the intracellular .gamma.-polyglutamylation of the agents. These biochemical parameters appear to be critical determinants for selective antitumor activity. In studies that document these differences, positions 5 and 10 on the classical antifolate-type molecular structure have been identified as sites where modification does not reduce binding to DHFR but does influence transport efficacy to favor inward flux into tumor cells and also intracellular .gamma.-polyglutamylation resulting in greater accumulation in tumor cells than in normal cells.
The above findings were exploited in the design of the 10-deazaaminopterin series of antifolates (DeGraw et al, J. Med. Chem., 17, 552 [1974]; DeGraw and Sirotnak, U.S. Pat. No. 4,393,964). This series, particularly 10-ethyl-10-deazaaminopterin (reviewed in DeGraw et al, J. Med. Chem., 29, 1056 [1986]), exhibited markedly enhanced therapeutic selectivity compared to MTX in both animal solid tumor models and human tumor xenografts (Schmid et al, Cancer Treat. Rep; 69, 551 [1985]). In on-going clinical trials, 10-ethyl-10-deazaaminopterin has shown significant therapeutic activity against non-small cell lung cancer (Shum et al, J. Clin. Oncol., 6, 446 [1988]; Kris et al, Proc. of ASCO, San Francisco, May 21-23, 1989; Abstr No. 884).
Current studies with the 5-deaza analogues of AMT and MTX (Piper et al, J. Med. Chem., 29, 1080 [1986] and U.S. Pat. No. 4,725,687) have revealed that 5-alkyl-5-deaza analogues of AMT and MTX are more active in vivo than MTX against four murine tumor models (Sirotnak et al, Cancer Res., 48, 5686 [1988]).
5,10-Dideazaaminopterin is disclosed by Taylor et al in J. Med. Chem., 28, 914-921 (1985) and by DeGraw et al in J. Heterocyclic Chem., 23, 1-4 (1986). 5-Methyl-5,10-dideazaaminopterin is disclosed by Piper et al in J. Med. Chem., 31, 2164-2169 (1988). Neither 5,10-dideazaaminopterin nor 5-methyl-5,10-dideazaaminopterin showed evidence of biological activity significantly greater than that of MTX.
U.S Pat. No. 4,684,653 to Taylor et al discloses compounds having the formula ##STR2## where R is defined as hydrogen, methyl or ethyl as antineoplastic agents.